Biomedical Engineering Reference
In-Depth Information
of the high-frequency oscillator increases at a certain phase of the low-frequency
oscillator. Such relationship between oscillators with different frequencies is called
cross-frequency coupling [
11
], which is the main topic of this chapter.
The aim of this chapter is to demonstrate that MEG source space analysis is
able to detect such cross-frequency interactions. It is a widely accepted notion that
low-frequency oscillations, such as alpha, are “encoders” of information processing,
and high-frequency oscillations, such as gamma, represent particular neuronal activ-
ities. Therefore, analysis of cross-frequency interactions between different cortical
regions would be extremely important for the investigation of information processing
mechanisms in a human brain.
9.2 Types of Cross-Frequency Coupling
Connections between brain oscillators at the same frequency, termed as same-
frequency coupling (SFC) in this chapter, have been investigated in various functional
connectivity studies. In such studies, the coherence (or phase-locking value) between
target oscillations is commonly used as a measure of connectivity.
1
In contrast to
SFC, the cross-frequency coupling (CFC) is a phenomenon in which oscillations
with different frequencies interact with each other. In general, CFC is classified into
the following coupling types between the oscillators:
•
Phase-amplitude coupling (PAC): The phase of a low-frequency (LF) oscillation
drives the amplitude of a high-frequency (HF) oscillation with the highest ampli-
tude of the HF oscillation occurring at a specific phase of the LF oscillation, which
is referred to as the “preferred coupling phase”.
•
Amplitude-amplitude coupling (AAC): The amplitude of an oscillation correlates
the amplitude of other oscillations. Even if the oscillators are not directly coupled,
their amplitude can be co-modulated.
•
Phase-phase coupling (PPC): The phase of an LF oscillation is related to the phase
of an HF oscillation. This is sometimes referred to as “
n
:
m
phase locking” where
phase locking occurs between
n
cycles of the HF oscillation and
m
cycles of the
LF oscillation.
Figure
9.1
depicts typical time courses of oscillations when a coupling exists.
When a pair of oscillations is interacting with each other, the interaction causes
a constant phase relationship between the two HF oscillations, resulting in same-
frequency coupling. This situation is depicted in Fig.
9.1
a. When phase-amplitude
coupling exists between two oscillators, the phase of the LF oscillation regulates the
amplitude of the HF oscillation. In this case, the HF oscillation increases in amplitude
at a certain phase of the LF oscillation. Such a situation is depicted in Fig.
9.1
b. One
special case of the PAC is that, when a common LF oscillation simultaneously drives
the amplitudes of two HF oscillators, amplitude-amplitude coupling (AAC) between
1
